A decade or so ago the Guardian ran an advertising campaign with the slogan "freethinkers welcome" - for some people, the appearance, on May 1, of three uncomplimentary, articles about Boris Johnson represented a crude attempt, on election day, to interfere with their thinking about who should be London's mayor. "Hmm, after five pages of hysteria I almost feel like voting for him," said one reader.

The articles were published as a trio in G2, the Guardian's features section. A piece, by columnist Zoe Williams, headlined: Be afraid. Be very afraid, was followed by quotes from celebrities and others - all negative - about the prospect of Johnson becoming mayor. Published alongside them was a selection of Johnson's quotes on subjects like race, homosexuality and sex (his gaffes). The G2 cover was a picture of Johnson looking askance with the strapline: "We know what London is. Boris is not London."

On the inside pages the paper didn't pull its punches. In a header introducing the features, it said: "Unbelievable as it may seem, Boris Johnson has a real chance of being elected London mayor today. Zoe Williams and other Londoners imagine what it would be like if this bigoted, lying, old Etonian buffoon got his hands on our diverse and liberal capital."

The 25 readers who complained didn't hold back either: "vitriolic in the extreme", "vicious", "outrageously partisan", "propaganda", "irresponsible" and "character assassination" were some of the things they said. Several were unhappy about the personal nature of Williams's attack: "That floppy hair ... that sodding bicycle," she wrote. Johnson, she said, was a "moneyed creep" and a "snob".

"I'm not a reporter," Williams points out. "I write comment. I tell people what to do all the time. I don't expect them to take me seriously." Some of her remarks, about Johnson's appearance and background, were gratuitous, but for the most part she made substantive criticisms that were pinned to things he'd said and done. You may not like the style and tone, but this was common or garden polemic.

The remarks, at the beginning of the article, about Johnson's hair and cycling were meant to be funny Williams told me. "Maybe I didn't demarcate my tones clearly enough," she said. "I went from joking into quite a trenchant attack." Some readers liked the piece, five took the trouble to write to the paper to say so and Williams tells me that she got more positive than negative feedback from people who contacted her directly.

British newspapers are by nature, habit and tradition partisan; in leaders and opinion pieces, writers wear their hearts on their sleeves. It follows that readers choose newspapers that reflect their own views: "Every newspaper ... is calculated for a particular set of readers only; so that if each set were to change its favourite publication for another, the communication would produce disgust and dissatisfaction to all," said John Walter in 1785, in the inaugural edition of the Times or the Daily Universal Register as it was then known.

Readers know this. So what was it about these articles that bothered some of them so much? The complaints suggest that while Guardian readers are comfortable with leader items and strongly worded opinion pieces, they don't appreciate the paper telling them what to think, or worse, how to vote in other articles. The timing of these pieces contributed to that impression. "[The Guardian] allowed itself to serve as a mouthpiece for a nasty personal attack - on election day no less," said one reader. "I'm not a Boris supporter," said another "but I take huge exception to newspapers telling me how to vote."

The intention wasn't to influence voters, G2's editor told me. "Most of our readers are not Tory voters," she said. "We wanted to reflect the fact that most readers would probably be quite alarmed by the idea of London having a Tory mayor." There was an assumption that because the articles appeared in the features section, rather than the main paper, people would appreciate the irreverent tone. "It was supposed to be fun, in a serious way," she said.